The goal of this project is to elucidate the mechanisms by which EYA1 controls nephron progenitor cell development in the mammalian kidney. In mammals, kidney development involves the specification of the metanephric mesenchyme (MM), the outgrowth of the ureteric bud (UB), and reciprocal interactions between the UB and the MM to generate large numbers of nephrons as well as the renal collecting system in the mature kidney. Mutations in genes that disrupt these processes cause renal agenesis and hypoplasia. We and others identified EYA1 and its cofactor SIX family proteins as critical mesenchymal factors for kidney development. Defects in these genes in humans cause Branchio-Oto-Renal (BOR) syndrome, a congenital birth defect characterized by a combination of branchial, otic and renal anomalies. To understand the basis of kidney defects that occur in BOR syndrome, we analyzed EYA1 and SIX1/5 mutations identified from patients and generated knockout mice. Our results show that these genes interact and are essential for MM cell development and UB branching. Since the MM is not formed in Eya1-/- mice, we have recently generated Eya1flox and Eya1CreERT2 alleles. We found that deletion of Eya1 after UB outgrowth results in depletion of the MM progenitors due to premature epithelialization and that EYA1 directly interacts with SIX2 and Myc family proteins, which are known to be important for the maintenance of the MM progenitors. Our analyses indicate that EYA1 dephosphorylates Myc at Thr58 to prevent the Myc protein from degradation in vitro and that deletion of Eya1 in the MM cells leads to increased levels of phosphor-T58-Myc but decreased levels of Myc. Currently, however, we do not understand how these key factors are functionally linked to regulate the proliferation and maintenance of the nephron progenitors. In this renewal application, we propose to take a combination of molecular, biochemical, genetic and genomic approaches to test the hypothesis that EYA1, Myc and SIX2 interact to regulate the expansion of the nephron progenitor pool and identify the target genes that are controlled by EYA1 and its cofactors. This study should advance the field by revealing novel mechanisms of how EYA1 interacts with its cofactors to regulate self-renewal of the nephron progenitors.